Many finding aids include a "Related materials in this repository" and/or "Related materials in other repositories" section which will direct you to related collections you might not know about!
Not every individual item is listed in a collection's finding aid. Many Special Collections/archival libraries do not catalog their collections by individual item.
Not every collection will have an online finding aid. While most institutions are working to get finding aids online, this is an ongoing process for many organizations. You may need to contact an archivist to learn more about which finding aids are available online, and which are not.
Morna Gerrard, Women and Gender Collections Archivist, Special Collections & Archives
Note: If you have questions about specific archival collections held at another repository (that is, not at GSU), you should contact that repository directly.
Pay special attention to the Scope and Content Note section of a finding aid. It will provide an overview of what materials are in the collection and alert you to weaknesses or gaps in the collection. You may discover that though a collection looks relevant to your research, the actual collection may be missing the items that you particularly need.